This invention relates to an ultrasonic motor and more particularly, to an ultrasonic motor which is driven by exciting ultrasonic vibration on a piezoelectric vibrator as a stator and rotating a rotor which is contact-pressed upon the piezoelectric vibrator via a frictional force therebetween.
An ultrasonic motor is a motor based on a novel principle and is given rotational force from ultrasonic vibration. Compared to the conventional electromagnetic motors based on the interaction between the electric current and magnetic fields, it has such entirely different features as low speed and high torque. More particularly, the torque per unit volume is extremely large, and the motor can be driven without gears and brakes if it is used at low speed. Another feature is that its response is superior as its inertial mass is small and the static retention of the rotor is large.
There have been proposed various ultrasonic motors, and researches have actively been made on them. For instance, a traveling wave type ultrasonic motor utilizes deflection vibration which is generated on the circumference of a stator comprising a piezoelectric ceramic plate attached on an elastic disk. The traveling waves which are traveling chronologically are excited on the stator by either two-phase or multiphase driving method to rotate the rotor pressed tightly on the surface of the stator. However, these traveling wave type ultrasonic motors have a large compliance of the stator compared to the rotor as they use bending mode traveling waves. Such motors therefore cannot easily obtain high driving force (e.g. torque of 1 Kgf-cm or higher with the diameter of 2 cm) while it can achieve middle range of the driving force relatively easily. Further the degree of freedom in design is small as the ultrasonic motors of bending mode traveling wave type generate driving force only with resonance.